Paleosols and paleoenvironments of early Mars Gregory J. Retallack Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA ABSTRACT METHODS Fluviolacustrine sediments filling Gale Crater on Mars show two levels of former exposure Chemical data to a depth of 25 cm below the and weathering that provide new insights into late Noachian (3.7 ± 0.3 Ga) paleoenvironments Viking 1 lander on Chryse Planitia (McSween of Mars. Diagnostic features of the two successive paleosols in the Sheepbed member include and Keil, 2000) are plotted in Figure 3; the Sher- complex cracking patterns of surface dilation (peds and cutans), a clayey surface (A horizon), gotty meteorite is also plotted as a likely basaltic deep sand-filled cracks with vertical lamination (sand wedges), and replacive sulfate nodules parent composition (Varnes et al., 2003). Viking aggregated into distinct bands (gypsic By horizon) above bedded sandy layers (sedimentary C 1 analyses have such poor analytical precision horizon). Shallow gypsic horizon, periglacial sand wedges, and limited chemical weathering that they do not reveal significant weathering are evidence of a hyperarid frigid paleoclimate, and this alternated with wetter conditions for after Gaussian error propagation for molecu- the lacustrine parent materials in Gale Crater during the late Noachian. Depletion of phos- lar weathering ratios (see Table DR1 [trans- phorus, vesicular structure, and replacive gypsic horizons of these Martian paleosols are fea- fer functions] in the GSA Data Repository1). tures of habitable microbial earth soils on Earth, and encourage further search for definitive Stratigraphic levels of analyses and their order evidence of early life on Mars. km INTRODUCTION 4000 km -8 0 +8 Based on an unprecedented stream of sci- Figure 1. Widely sepa- entific information from two active rovers on Utopia Planitia rated sites of similar pa- Mars (Arvidson et al., 2014; Grotzinger et al., (Viking 2) leosols on Mars, such as Mawrth Vallis Yila pedotypes at Mati- 2014), this paper advances a new interpreta- Chryse Planitia (Viking 1) (unvisited) jevic Hill and Yellowknife tion that Martian outcrops include fossil soils; Matijevic Hill Yellowknife Bay, and Spender pedo- this has novel implications for past Martian (Opportunity) Bay(Curiosity) types at Chryse and Uto- habitability. Soils are sometimes defined by pia Planitiae. Base map is biological activity, but an alternative defini- Mars Orbiter Laser Altim- eter (MOLA) global topo- tion also has a following, i.e., soils as plan- graphic map of Mars; etary surfaces altered in place by biological, names of spacecraft or chemical, or physical processes (Retallack rovers are in italics. 2001; Amundson et al., 2008). By this defini- tion, the question is not whether the surface of Mars has soil, but whether Martian soil is or Figure 2. Paleosols of was alive. There is no current indication of life the Sheepbed member, on Mars, and surface soil profiles at Chryse Yellowknife formation, and Utopia Planitiae explored by the Viking Gale Crater, Mars. A: missions are too clayey for current hyper- Analytical stations and their stratigraphic order arid and hyperfrigid Mars surface conditions (McLennan et al., 2014): (Amundson et al., 2008), so were relict paleo- 1—Cumberland Brush, sols (Retallack, 2001). Nevertheless, Chryse 2—Mavor, 3—Persillon, and Utopia Planitiae paleosols reveal much 4—Brock Inlier, 5—Nas- tapoka, 6—Drill RP, 7— about early (3.5 ± 0.5 Ga; Hartmann and Neu- Drillhole R4, 8—McGrath kum, 2001) and late Hesperian (3.2 ± 0.6 Ga; R3, 9—Wernecke Brush, Thomson and Schultz, 2007), respectively, 10—Divot 2, 11—Sayunei surface conditions on Mars. New documenta- C, 12—Bonnet Plume, tion of late Noachian (3.7 ± 0.3 Ga) fluviola- 13—Hudson Bay, 14— Hay Creek, 15—Ekwir custrine rocks (Thomson et al., 2011) of the 1 Brush, 16—Grit (1–16 Sheepbed member of the Yellowknife Bay are Sheepbed member), formation within Gale Crater (Grotzinger et 17—Ungava (in overly- al., 2014) now reveal geochemical (McLen- ing Gillespie member), 18—Rocknest (Glenelg nan et al., 2014) and petrographic (Vaniman member in distance). et al., 2014) details of possible buried paleo- Note prominent rock sols on Mars (Figs. 1 and 2), previously in- rib (“snake”) of basal- ferred only from remote sensing (Horgan et tic sand. B: Interpreted al., 2012). These paleosols are a new line of soil horizons. Lenticular regions of low nodule evidence for late Noachian paleoclimate and density in horizon By are habitability of Mars. considered parent mate- rial irregularities. *E-mail: [email protected]. 1GSA Data Repository item 2014275, Table DR1, transfer functions and error propagation, is available online at www.geosociety.org/pubs/ft2014.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, September 2014; v. 42; no. 9; p. 755–758; Data Repository item 2014275 | doi:10.1130/G35912.1 | Published online 14 July 2014 GEOLOGY© 2014 Geological | September Society 2014of America. | www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 755 Al2O3 Al O A. Chryse Planitia CaO+MgO 2 3 CaO+MgO+ SO3 Grain size (%) Minerals (%) Al O cm. Al2O3 SiO2 Na O+K O 2 3 silt gravel 50 sulfate opaque 2 2 0 clay sand 50 A By sand C 20 clay type Spender clay Spender type silt Figure 3. Molar weather- clay and amorphous Shergotty Shergotty ing ratios of paleosols at meteorite meteorite Chryse Planitia (A) and gravel pyroxene quartz 2460.050.1 0.015 0.10.2 0.30.4 123 Gale Crater (B), Mars. feldspar molar ratios Al2O3 Viking 1 rover results for B. Yellowknife Bay Na2O CaO+MgO Al2O3 CaO+MgO+ SO the late Hesperian (3.5 Grain size (%)Minerals (%) 3 quartz K2O Al2O3 SiO2 Na2O+K2O Al2O3 ± 0.5 Ga) Spender paleo- clay silt sand gravel 50 50 sols of Chryse Planitia cm. sand are compromised by low analytical precision, but 0 vein-rich A vein-rich not results from late Noa- clay ("Mavor") ("Mavor") chian (3.7 ± 0.1 Ga) Yila silt paleosols analyzed by By feldspar Curiosity rover in Gale l ila silty clay clay silty ila Crater, where pedogenic Y C clay formed largely at the clay expense of olivine, and A size clay sulfates accumulated. Mavor sample is plotted separately because it is 50 By silt amorphous vein rich. feldspar ila silty clay silty ila Y type type C 6810 12 4680.100.14 0.15 0.20.25 24 6 salinization calcification clayeyness base loss gypsification gravel sulfate pigeonite sulphate clayey olivine augite nodules clasts basaltic basaltic siltstone, planar ferruginized greenish- sulfate- boulders sandstone shale bedding surface gray halo filled cracks in Yellowknife Bay are from McLennan et al. contact of the Sheepbed member overlain by the Huang, 2010). A prominent rock rib (“snake”, (2014) and Grotzinger et al. (2014). Mineral sandy Gillespie member, which shows scour- Fig. 2A) is basaltic sand with clasts of finer- compositions were reconstructed by Vaniman ing and bedding, incorporating likely clasts of grained sediment and vertical bedding, centered et al. (2014) for the Sheepbed member in bores the underlying Sheepbed member (Grotzinger on an uparched arched surface of the Sheepbed from Cumberland and John Klein localities, et al., 2014). Bedding is strongly disrupted by member (Grotzinger et al., 2014). Uparching, and for overlying Gillespie and Glenelg mem- a system of cracks and veins, widest at the top taken as evidence of upward intrusion by Grotz- bers from Rocknest outcrop. Drilling behavior of the Sheepbed member, but also seen at other inger et al. (2014), as well as other features of at Yellowknife Bay suggested a bulk density levels in the member where intervals of dila- the “snake” are comparable with periglacial comparable with Pliocene siltstone from Cali- tional deformation are overlain by thin beds of sand wedges on Earth (Williams, 1986). fornia (USA) (Grotzinger et al., 2014). Paleo- sandy sediment (Grotzinger et al., 2014). These Paleosols were not recognized by Grotzinger sols at Yellowknife Bay may have been buried dilational cracking systems have coatings of sul- et al. (2014) or Schieber et al. (2013), who re- by as much as 5.2 km of overlying rocks on fate, comparable with soluans of desert soils on garded the outcrop as fluviolacustrine facies Mount Sharp (Thomson et al., 2011), and like Earth (Amundson et al., 2008), and also define cracked by hydraulic fracturing or synaeresis, other paleosols buried as deeply (Retallack, blocky angular ped structure of clayey surface and infiltrated with diagenetic nodules. Diagen- 2012), would have had a relatively uniform (A) horizons (Retallack, 2001). Disruption of esis includes alteration after deposition as well bulk density compared with original soil. The lacustrine lamination characteristic of soils in- as after burial, and it is only the latter that differs density for mass balance analysis (Brimhall cludes abundant nodules of the Sheepbed mem- from the paleosol interpretation presented here. et al., 1992) used here (Fig. 4) assumed a uni- ber (Grotzinger et al., 2014). Hollow nodules Nodules of the Sheepbed member are small, form bulk density for all samples of 2.33 g may have been gas vesicles (Grotzinger et al., complex, and dispersed like soil nodules (Retal- cm–3 from Pliocene Gelisols (Gypsic Anhytur- 2014), comparable with vesicular structure com- lack and Huang, 2010), unlike burial diagenetic bels) of Antarctica (Retallack et al., 2001). mon in desert soils on Earth (McFadden et al., nodules that are large, rounded, and less strati- 1998). Nodule composition is similar to that of graphically restricted (Retallack, 2001). Hy- PALEOSOL RECOGNITION the matrix, but enriched in Fe, Ca, Cl, Br, Ni, draulic fracturing and synaeresis are unlikely, Fluviolacustrine sediments found by Grotz- and Ge, suggesting cements of iron oxyhydrox- considering ptygmatic folding of a deep sulfate inger et al.